best way to insulate

okay here are some views of the area i am trying to insulate. this is my shop when i finish remodeling it. Yes i want i nice and comfy inside when i am in there. but i don't want the tar on the roof to melt. and i don't know how easy/practical it will be to cut a hole in the roof and install vents. but i can try if that is going to be the best thing i can do.

Spray Foam is not the answer to that. I just got done with my local building dept. This is what they have told people OK foam it Now how is your roof going to breath? second if you have a roof leak how if the sheathing going to dry out? things to think about. That would be a good aplication of blown in and call it good. Out on the coast of washington cellulose is not as good to use to much moist air. But were your at use cellulose and call it good.

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17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?

I wouldn't suggest any type of insulation that would allow convective loops. Closed cell spray foam is my recommendation

OK put it in there. then tell us how the udnerside of the sheathing is to breath? with out that air exchange the roof fails. what happens when the roof gets a leak? were is the water going to go? and when the roof is repaired how is the sheathing going to dry out? foaming the underside of the roof is a code violation in washington state. His problem can be solved with 10inches of blown in cellulose. call it good.

10 inches of blow in cellulose that would hold moisture with the air movement through it, unless it is truly, successfully densepacked, which is extremely difficult with that much depth, which would give you the same performance as spray foam as far as air movement, with less r-value.

If breathability of the roof deck is the main concern, put 1x2" spacers attached to the rafters where they meet the deck, and put fanfold there, and apply the sprayfoam to that. It will allow plenty of air movement behind the insulation, give you your best finished product.

In this given application, any negatives applied to spray foam also apply to cellulose and then some. Read the entire link that you liked from gary's post above. It goes into detail about the exact issues I am discussing.

Entry 1:
No one argues that it is a dumb idea to shove a bunch of fiberglass batts into a cathedral ceiling with no provision for ventilation air to remove moisture that leaks in from the interior. The results are often catastrophic (Photograph 6). No matter how good we are at air sealing the interior gypsum board ceiling we can’t quite get it “perfect enough” consistently to stay out of trouble. We can get lucky on one in 10, but that is not a high success rate. I mean it is possible to get a blindfolded drunk to cross Niagara Falls on a high-wire without a net, but it wouldn’t be a good idea.8 Note that not only did we insulate the roof cavity, but we also did our best at air sealing as well with caulks and foams and whatever, and it still did not work. So, what makes us think that we can dense pack a cathedral ceiling and get it to work without any attempt at air sealing?

Entry 2:
So, what should we do with these flat roofs and cathedral ceilings that can’t be vented and insulated in the “typical” way? Simple, follow the building code. Add insulation on the top of the deck to elevate the roof deck temperature above the dew point of the interior air vapor mix (Figure 1) before dense packing or take the interior ceiling down and install an air impermeable insulation. Spray foam on the underside of the roof deck (Figure 2). The thermal resistance of the insulation needed to be added above the roof deck or in the form of spray foam on the underside of the roof deck is dependent on climate and the interior moisture load. Tables and a map in the International Residential Code make it easy for you.10 The building code also lets you know what a “desert” looks like and where “deserts” are located and under what conditions you don’t need to vent and can in fact dense pack. But, if you are not in a desert with no top side vapor barrier, just say no to dense packing a roof.

And I am aware this is not a cathedral ceiling per say, but given the HIGHEST point is only a foot, it should be treated as one for the most part.

it is his shop not his home. 8 inches of Cellulose will give him almost a R 30 value. if he has bird blocking in place he needs to put baffles in he lives in New Mexico dry arid climate. Cellulose is perfect for that enviroment. I am by no means a fan of cellulose but it does have its place. This is one of them.

It is ideal to allow the sheathing to have some air exchange while I know that the SPF community (and I partially agree) contends that the sheathing doesn't need to breath.

If it is a shop and you are just looking for some help from the heat, a radiant barrier tacked up to the underside of the roof framing, with some venting (soffit and exhaust), some batts under the radiant barrier and across the ceiling, will give you pretty good performance.

If you were going to be living there, rigid foam board with a vent space and SPF would work great.